With the progress of small-size, high-speed and lightweight electronic devices such as cellular phones and personal computers, the capacitor used for these electronic devices is demanded to have a smaller size, a lighter weight, a larger capacitance and a lower equivalent series resistance (ESR).
As an example of such a capacitor, an electrolytic capacitor has been proposed, which capacitor is produced by anodically oxidizing a sintered body (anode body) obtained by sintering valve-acting metal powder such as tantalum which can be anodized to form a dielectric layer made of the oxide of the metal on the surface of the sintered body.
The electrolytic capacitor using tungsten as a valve-acting metal and employing a sintered body of the tungsten powder as an anode body can attain a larger capacitance compared to the electrolytic capacitor obtained with the same formation voltage by employing an anode body of the same volume using the tantalum powder having the same particle diameter but have a problem of high leakage current (LC).
The present applicant found that the problem of the LC characteristics can be solved by using a tungsten powder comprising a specific amount of tungsten silicide in the particle surface region, and proposed a tungsten powder comprising tungsten silicide in the particle surface region and having a silicon content of 0.05 to 7 mass %; an anode body comprising the sintered body of the tungsten powder; an electrolytic capacitor; and a production method thereof (Patent Document 1; European Patent No. 2656947).
The above-mentioned tungsten electrolytic capacitor is manufactured by forming a dielectric layer on the surface layer of an anode body obtained by molding a powder mainly comprising tungsten, followed by sintering, and sequentially forming a semiconductor layer and a conductor layer on the dielectric layer. Here, a tungsten anode body before the formation of a dielectric layer is left in the air, a natural oxide film is likely to be formed on the surface of the anode body. Compared to electrolytic oxide films formed by chemical conversion, a natural oxide films are uneven in thickness and thus have inferior withstand voltage characteristics. Therefore, an etching process is conducted prior to the chemical conversion treatment process to form a dielectric layer on the anode body so as to entirely remove the natural oxide film having an uneven thickness present on the surface of the anode body in order to form a dielectric layer having an even thickness. However, cracks are generated in the dielectric layer in some cases depending on the thickness of the dielectric layer formed by the chemical conversion treatment, leading to a problem of an uneven thickness of the cathode layer (an organic semiconductor layer formed by polymerization) formed on the dielectric layer in a subsequent process, which causes deterioration in leakage current (LC) as a result.
As the prior art relevant to the etching method to remove a natural oxide film on the surface of the tungsten anode body, which method is employed in the present invention, for example, Patent Document 2 (JP 2009-177174 A (U.S. Pat. No. 7,768,773)) discloses a method of etching an oxide film on the surface of a niobium sintered body with an organic acid; Patent Document 3 (JP 2007-273839 A) discloses a method of etching an oxide film on the surface of an aluminum foil with a mixed solution of phosphoric acid and chromic acid; and Patent Document 4 (WO 2013/190756) discloses a method for entirely removing a natural oxide film by passing current through a tungsten anode body in an alkaline solution.